Various acute and chronic conditions and diseases originate from excitable tissue damage and dysfunction brought about by external and internal stimuli. Such stimuli include lack of adequate oxygenation or glucose, neurotoxins, consequences of aging, infectious agents, and trauma. For example, excitable tissue may be subjected to damage as a consequence of seizures and chronic seizure disorders, convulsions, epilepsy, stroke, Alzheimer's disease, Parkinson's disease, central nervous system injury, hypoxia, cerebral palsy, brain or spinal cord trauma, AIDS dementia and other forms of dementia, age-related loss of cognitive function, memory loss, amyotrophic lateral sclerosis, multiple sclerosis, hypotension, cardiac arrest, neuronal loss, smoke inhalation and carbon monoxide poisoning.
It is widely understood that decreases in energy supply available to the brain, such as glucose or oxygen, results in a profound impairment of brain function, including cognition. Many (but not all) neurons in the central nervous system are easily damaged while working under metabolically-limited conditions, e.g., hypoxia, hypoglycemia, stress, and/or prolonged, strong excitation. Under these circumstances, the electrochemical gradients of these cells often collapse, resulting in irreversible neuronal injury and cell death. Current opinion favors this general mechanism as a common final pathway for a wide range of common and debilitating degenerative neurological diseases including stroke, epilepsy, and Alzheimer's disease.
Although the consequences of limited energy substrate on brain function are well known, the effects of improving energy delivery in an otherwise normal brain has been less extensively explored. Current data suggest strongly that improved delivery of either glucose or oxygen markedly improves complex cognitive function in both animal models and in normal human subjects (Kopf et al., 1994, Behavioral and Neural Biology 62:237-243; Li et al., 1998, Neuroscience 85:785-794; Moss et al., 1996, Psychopharmacology 124:255-260). Further, a growing list of neuropeptides produced within the brain have been demonstrated to directly provide an improvement in cognitive function in normal brain. The physiological basis of these enhancements ultimately depends upon remodeling of neuronal interconnections through synaptic changes.
Brain tissue cytoarchitecture exhibits extreme plasticity and undergoes continuous remodeling. These processes, mediated by many trophic molecules, occur not only following injury, but also play a prominent role in learning, memory, and cognitive function. Although the prototype neurotrophin is nerve growth factor (NGF), an increasing number of cytokines have been recognized to perform trophic functions in the brain (Hefti et al. 1997, Annu. Rev. Pharmacol. Toxicol. 37:239-67).
Recently, a number of independent investigators have recognized that nervous tissue expresses high levels of both EPO and its receptor (EPO-R; Digicaylioglu et al., 1998, Proc. Natl. Acad. Sci. USA 92:3717-20; Juul et al., Pediatr. Res. 43:40-9; Marti et al., 1997, Kidney Int. 51:416-8; Morishita et al., 1997, Neuroscience 76:105-16). Although it appears that EPO and its receptor proteins are each the products of single genes, the CNS versions are significantly smaller. The physiological meaning of this observation has not been clarified, but the mass differences do appear to modify biological activity. For example, in studies of human patients, investigators have concluded that EPO is not transported into the brain from the periphery (Marti et al., 1997, supra). To date, however, this possibility has not been evaluated for EPO by any direct study. Although brain EPO is about 15% smaller than renal EPO (due to differences in sialylation), brain EPO is more active in erythroid colony stimulation at low ligand concentrations (Masuda et al., 1994, J. Biol. Chem. 269:19488-93). On the other hand, the CNS receptor exhibits a much lower affinity for deglycosylated EPO than the 30% larger peripheral receptor (Konishi et al., 1993, Brain Res. 609:29-35; (Masuda et al., 1993, J. Biol. Chem. 268:11208-16).
In the brain, EPO expression has been found in astrocytes, and increased EPO expression and release can be induced by hypoxia and other metabolic stressors (Marti et al., 1996, Eur. J. Neurosci. 8:666-76; Masuda et al., 1993, J. Biol. Chem. 268:11208-16; Masuda et al., 1994, J. Biol. Chem. 269:19488-93) or even by occupancy of other receptors such as insulin-like growth factor family (Masuda et al., 1997, Brain Res. 746:63-70). Neurons are one target for this secreted EPO as they express EPO-R in a highly cell type-specific manner (Morishita et al., 1997, Neuroscience 76:105-16). In contrast to EPO itself, EPO-R density does not appear to be modulated during metabolic stress (Digicaylioglu et al., 1995, Proc. Natl. Acad. Sci. USA 92:3717-20).
Recent study has demonstrated that EPO impressively protects against hypoxic neuronal injury in vitro, as well as in vivo when injected directly into the cerebral ventricles (Morishita et al., 1997, Neuroscience 76:105-16; Sadamoto et al., 1998, Biochem. Biophys. Res. Commun. 253:26-32; Sakanaka et al., 1998, Proc. Natl. Acad. Sci. USA 95:4635-40). Konishi et al. (1993, Brain Res. 609:29-35) have demonstrated that EPO promotes the in vivo survival of cholinergic neurons in adult rats when injected directly into the cerebral ventricles. EPO administered centrally into the cerebral ventricles also successfully prevents ischemic injury-related deficits in spatial learning in rats (Sadamoto et al., 1998, Biochem. Biophys. Res. Commun. 253:26-32). A recent publication suggests that only a 17-amino acid portion of EPO is needed for these neurotrophic effects in cultured neural cells (Campana et al., 1998, Int. J. Mol. Med. 1:235-41).
For many years, the only clear physiological role of erythropoietin (EPO) had been its control of the production of red blood cells. Recently, several lines of evidence suggest that EPO, as a member of the cytokine superfamily, performs other important physiologic functions which are mediated through interaction with the erythropoietin receptor (EPO-R). These actions include mitogenesis, modulation of calcium influx into smooth muscle and neural cells, and effects on intermediary metabolism. It is believed that EPO provides compensatory responses that serve to improve hypoxic cellular microenvironments. Although studies have established that EPO injected intracranially protects neurons against hypoxic neuronal injury, intracranial administration is an impractical and unacceptable route of administration for therapeutic use, particularly for normal individuals. Furthermore, previous studies of anemic patients given EPO have concluded that peripherally-administered EPO is not transported into the brain (Marti et al., 1997, supra).
Citation or discussion of a reference herein shall not be construed as an admission that such is prior art to the present invention.